fix(webapp): prevent IEEE 754 precision loss on OTLP nanosecond conversions

Fixes #3292

Root Cause:
Converting epoch milliseconds to nanoseconds via `BigInt(ms * 1_000_000)` performs
multiplication in IEEE 754 float representation before casting to BigInt. Because
epoch ms * 1e6 is ~1.7e18 and exceeds Number.MAX_SAFE_INTEGER (~9e15), the lowest
bits lose precision, creating +-256ns non-deterministic drift in span ordering and
duration metrics.

Resolution:
Cast epoch ms to BigInt before multiplying by BigInt(1_000_000) across all webapp
event repository and run engine call sites.

Author: Ravi Patel

apps/webapp/app/v3/eventRepository/common.server.test.ts

@@ -0,0 +1,25 @@
+import { describe, it, expect } from "vitest";
+import { getNowInNanoseconds, calculateDurationFromStart } from "./common.server";
+
+describe("OTLP nanosecond precision", () => {
+  it("guarantees exact integer arithmetic without float truncation", () => {
+    const sampleMs = 1729200000123;
+    const sampleDate = new Date(sampleMs);
+    const expectedNano = BigInt(sampleMs) * 1_000_000n;
+
+    // Buggy float math produces 1729200000123000064n (+64ns error)
+    const buggyNano = BigInt(sampleMs * 1_000_000);
+    expect(buggyNano).not.toBe(expectedNano);
+
+    // Fixed calculation produces exact 1729200000123000000n
+    const fixedNano = BigInt(sampleDate.getTime()) * BigInt(1_000_000);
+    expect(fixedNano).toBe(expectedNano);
+  });
+
+  it("preserves exact duration calculation", () => {
+    const start = 1729200000000n * 1_000_000n;
+    const end = new Date(1729200000500); // 500ms later
+    const duration = calculateDurationFromStart(end, start);
+    expect(duration).toBe(500_000_000);
+  });
+});

apps/webapp/app/v3/eventRepository/common.server.ts

@@ -26,7 +26,7 @@ export function extractContextFromCarrier(carrier: Record<string, unknown>) {
 }
 
 export function getNowInNanoseconds(): bigint {
-  return BigInt(new Date().getTime() * 1_000_000);
+  return BigInt(new Date().getTime()) * BigInt(1_000_000);
 }
 
 export function getDateFromNanoseconds(nanoseconds: bigint): Date {
@@ -40,7 +40,7 @@ export function calculateDurationFromStart(
 ) {
   const $endtime = typeof endTime === "string" ? new Date(endTime) : endTime;
 
-  const duration = Number(BigInt($endtime.getTime() * 1_000_000) - startTime);
+  const duration = Number(BigInt($endtime.getTime()) * BigInt(1_000_000) - startTime);
 
   if (minimumDuration && duration < minimumDuration) {
     return minimumDuration;

apps/webapp/app/v3/eventRepository/index.server.ts

@@ -208,7 +208,7 @@ async function recordRunEvent(
         runId: foundRun.friendlyId,
         ...attributes,
       },
-      startTime: BigInt((startTime?.getTime() ?? Date.now()) * 1_000_000),
+      startTime: BigInt(startTime?.getTime() ?? Date.now()) * BigInt(1_000_000),
       ...optionsRest,
     });
 

apps/webapp/app/v3/runEngineHandlers.server.ts

@@ -557,7 +557,7 @@ export function registerRunEngineEventBusHandlers() {
         );
 
         await eventRepository.recordEvent(retryMessage, {
-          startTime: BigInt(time.getTime() * 1000000),
+          startTime: BigInt(time.getTime()) * BigInt(1_000_000),
           taskSlug: run.taskIdentifier,
           environment,
           attributes: {